1. Field of the Invention
The present invention relates to a zoom lens system having relatively high zoom ratios of about 3 ranging to a wide field angle. More specifically, the present invention is capable of providing a zooming range of 28 to 85 mm, 35 to 85 mm, 35 to 105 mm, and 45 to 135 mm lens systems that are suitable for 35 mm single lens reflex cameras, while sufficiently balancing the aberrations that occur over the entire range of zooming.
2. Description of the Prior Art
It is generally known in the prior art that a zoom lens system is not readily adaptable to a relatively high zoom ratio ranging to a wide angle, particularly when it is also desired to be a relatively lightweight compact lens system. It is also known that focusing in the zoom lens system is conventionally achieved by means of axial movement of the front lens group. This design choice is exercised in zoom lens systems because the degree of movement required of the focusing lens group to focus an identical object distance does not change with change in focal length. If, however, the refractive power of the front lens group is positive, then the diameter of the front lens group has to be increased to provide a desired wide field angle when it is moved towards the object side for focusing. The choice of a positive refractive power for the front lens group generally increases the total length of the lens system. If the lens designer selects a negative refractive power for the front lens group, then other problems are also created since it is difficult to increase the zoom ratio because of the necessary aberration correction, the lens system will not be as tolerant to manufacturing error, and the resulting lens system will generally be bulky.
Examples of patented prior art literature can be found in U.S. Pat. No. 3,481,666, U.S. Pat. No. 3,549,242, and U.S. Pat. No. 4,240,700. Additional prior art can be found in the Japanese Patent Application Laid Open No. 55-62419 of May 10, 1980, which claims respective priority dates from the U.S. patent application Ser. No. 941,430 (Sept. 11, 1978) now abandoned, and U.S. patent application Ser. No. 70,749 (Sept. 5, 1979) now U.S. Pat. No. 4,299,454. Finally, published advertisements and articles on the TOKINA 35-105 mm F/3.5-4.3 lens can be found in "Modern Photography", April 1980, and at pages 82-84 of the "Camera Review" of September 1980.
The prior art is still seeking to provide a relatively compact and economical zoom lens system having a relatively wide range of zooming into the wide angle range with adequate optical correction.